It is well known that the aesthetic desirability, corrosion resistance and paint adhesion properties of an aluminum surface may be improved by forming a chromate conversion coating by contacting the surface with an aqueous conversion coating composition containing hexavalent chromium ions, phosphate ions and fluoride ions. By way of illustration, such coatings are disclosed in U.S. Pat. Nos. 2,438,877; 2,928,763 and 3,377,212 and in British Patent Specification No. 891,910.
British Patent Specification No. 891,910 describes a process employing an aluminum coating composition containing 2 to 60 grams/liter of hexavalent chromium ion (calculated as CrO.sub.3), 0.15 to 12.5 grams/liter of fluoride ion and 2 to 285 grams/liter of phosphate ion. The pH of the disclosed composition is in the range of from 0.8 to 4.5.
U.S. Pat. No. 2,438,877 describes an aluminum coating process using a coating composition containing 3.75 to 60.0 grams/liter of hexavalent chromium ion (calculated as CrO.sub.3), 0.9 to 12.5 grams/liter of fluoride ion and 2 to 285 grams/liter of phosphate ion. The pH of this composition is between about 1.6 and 2.2.
U.S. Pat. No. 2,928,763 discloses an aluminum coating composition consisting essentially of from 2.5 to 62 grams/liter of hexavalent chromium ion (calculated as CrO.sub.3), from about 2.5 to 123 grams/liter of fluoride ion, from 5 to 150 grams/liter of phosphate ion, from 1 to 55 grams/liter of aluminum ion, and hydrogen ion. The pH of the composition is disclosed as being between 0.8 and 1.5.
U.S. Pat. No. 3,377,212 discloses an aluminum coating composition containing from 0.1 to 2.4 grams/liter of chromic acid (calculated as CrO.sub.3), 0.1 to 1.5 grams/liter of fluoride ion, and 1 to 15 ml./1. of 75% phosphoric acid (corresponding to about 1.2 to 17.8 grams/liter of phosphate ion). The composition disclosed in this patent represents what was heretofore considered to be the minimum concentrations of phosphate, chromate and fluoride ions that would produce an effective aluminum coating.
There has been growing concern in recent years about pollution of rivers and waterways, and attention has been directed to reducing or eliminating harmful materials from the effluent of industrial processes. Hexavalent chromium can cause problems if discharged into waterways because of its strongly oxidizing character. As a result, conventional chromate conversion coating processes have required extensive waste treatment procedures to eliminate possible harmful effects resulting from the discharge of hexavalent chromium. This necessarily results in increased costs, and as a result attention has been given to developing alternative processes for improving the characteristics of aluminum surfaces which employ no hexavalent chromium. In particular, processes have been developed using zirconium based coating compositions. However, these alternative processes have frequently proved more difficult to control with respect to treating solution composition than chromate coating solutions.